This invention relates to improvements in or relating to vertical type roller mills, and more particularly it is concerned with a vertical type roller mill having grinding rollers of a tire shape.
In one type of vertical roller mill having grinding rollers of the tire shape of the prior art, a plurality of grinding rollers of the tire shape are arranged for rotation on a grinding table which is located horizontally and driven for rotation, and a material to be ground is fed on to the grinding table and ground between the grinding rollers and the grinding table which are rotating relative to each other.
In this type of vertical type roller mill, the grinding rollers of the tire type used for performing grinding are symmetrical in shape so that they are identical in shape on both sides. Thus, this type of rollers offer the advantage that they have a higher rate of use because they can be used by reversing their sides. However, they are not without disadvantages. The center axis of the rotary shaft of each grinding roller does not intersect the center axis of the rotary shaft of the grinding table and the material grinding surfaces of the rollers differ from the material grinding surface of the table in peripheral velocity, resulting in a production of a relative displacement (relative slipping effect) between the grinding table and the grinding rollers. Thus, when a grinding operation is performed by feeding a material to the mill, the relative displacement causes wear of a higher magnitude on the side of each grinding roller disposed near the center of the grinding table which grinds the material into coarse particles than on the opposite side remote from the center of the grinding table. The reason why the wear caused on the side remote from the center of the grinding table which crushes the coarse particles into fine particles is low in magnitude in spite of a substantial relative displacement is because the material is already ground to some degree during grinding into coarse particles when it is supplied to the remote side of each grinding roller. Owing to the difference in the magnitude of wear caused on opposite sides of each grinding roller as described hereinabove, asymmetrical wear is caused on the grinding rollers and grinding table.
If the development of the asymmetrical wear is allowed to run its course, the amount of the material ground in a unit time will be reduced and the amount of untreated material circulating in the mill will increase. As a result, the grinding efficiency of the roller mill be reduced and the consumption of electrical power required for driving the mill will rise.